Spinal fusion surgery for degenerative disc disease involves removing the damaged disc and replacing it with bone grafted from another site on the patient's body, bone from a donor, or artificial or synthetic bone graft material that stimulates bone growth to fuse, or join, the two vertebrae together to stabilize the spine. In all spinal interbody fusion surgeries, disc material is removed. A spacer, referred to as a “cage” is then inserted into the disc space.
The fusion cages help separate the vertebral bodies, taking pressure off the spinal nerves, which travel from the spinal canal through openings, each called the neural foramen. The expansion pulls the ligaments inside the spinal canal taut so they don't buckle into the spinal canal and cause compression of the nerves. Surgeons monitor the position and correct placement of the cages using fluoroscopy and Electromyography (EMG) monitoring.
Fusion cages known in the art are most commonly made of metal, graphite, bone, or PEEK (polyether ether ketone). Many of these cages are shaped like cylinders. A few are rectangular in shape. The main purpose of the cage, regardless of the shape or material, is to hold the two vertebrae apart while the fusion becomes solid.
Generally, two cages are placed side by side within the disc space spreading the vertebrae apart. After implanting the cages, most surgeons attach metal hardware or screws to the vertebrae to rigidly lock them in place. This allows the bone graft to effectively fuse the vertebrae together.
The hollow center of the cage is packed with bone graft material, either in the form of natural bone taken from another site on the patient or from a donor or an artificial or synthetic bone substitute.
When bone is taken from another part of the patient's own body (i.e., autograft), there is a risk of pain, infection, or weakness in the area where the graft is taken. Synthetic bone growth alternatives offer an alternative to using the patient's own bone. Using gene therapy, scientists have produced bone graft substitutes (i.e., growth factors). These growth factors are natural proteins found in the human body. Genetic engineers have been able to clone proteins known as bone morphogenetic proteins (BMPs). These proteins are then made available as powder, small particles, or chips. Hormones that circulate in the bloodstream act on the BMP molecules, causing them to build new bone tissue.
The growth factor that is approved for lumbar fusion with titanium fusion cages is BMP-2. Substituting BMP-2 for an autograft eliminates complications and the recovery associated with harvesting autograft material from the patient's own body. One example of a commercially available bone growth material is Infuse® Bone Graft by Medtronic.
A risk associated with the use of bone growth material is that the nerves may be exposed to the material causing bone formations around or adjacent to the nerves, which can cause severe neurological injury or paralysis.
There are three different approaches for spinal fusion surgeries: anterior, posterior and lateral. Anterior interbody spinal fusion is performed via an incision in the patient's abdomen and the vertebral bodies are approached from the front. This approach is generally used when the surgeon needs to reach the front part of the spine. The abdominal muscles must be displaced resulting in considerable patient discomfort and increased recovery time. The use of bone formation material is currently approved by the FDA only for the anterior approach, because this approach reduces risk of exposing the lumbar nerves to the bone growth material.
Posterior interbody spinal fusion is performed from an incision made in the back. The posterior approach is necessary if a decompression procedure is performed in addition to a spinal fusion. The use of bone formation material poses considerable risk since the lumbar nerves are exposed during the procedure. Any displacement of the bone formation material can cause substantial nerve damage.
Lateral spinal fusion techniques have been gaining popularity. The procedure is performed through the patient's side, avoiding the major muscles of the abdomen and back. With recent advances in neurologic monitoring capabilities, surgeons are able to safely navigate around the lumbar nerves in order to enter the disc space laterally. However, synthetic bone growth material is not currently approved by the FDA for use in lateral spinal fusion procedures.
Each vertebra has a pair of transverse processes, one on each side of the spinal column. Spinal muscles attach to the transverse processes. The pedicle, a short projection of bone, lies between the back of the vertebral body and the transverse process and extends from the spinal column in the back to the vertebral body in front.
Pedicle screws can be used alone or in conjunction with bone cages. Using the “posterior approach,” pedicle screws are placed into the pedicles. Each patient's pedicles are of a different size, so the screws are available in different diameters and lengths. Two screws are placed into each vertebra (one in each of two pedicles).
A problem known in the art is that when a disc is removed and pedicle screws are inserted, there is a loss of support due to the displacement of the disc material that is normally in contact with the endplates. Displacement of disc material leaves a void and pressure formerly absorbed by the disc material is partially redistributed to a component held in place with pedicle screws. It is desirable to minimize the amount of force placed on the pedicle screws to avoid breakage of the pedicle screw system. It is further desirable to fill the void left by the removal of disc material in a manner that maximizes contact of the device (e.g., a bone cage known in the art) with the endplates. It is critical that any device placed in the space formerly occupied by the disc has effective contact with the endplates in order for the surgery to promote the fusion of the vertebrae and to avoid fracture of the endplates. This fusion process is the objective of the surgery, and the success of the surgery depends on how effectively fusion occurs.
A bone cage(s) is placed in the disc space while the pedicle screws are in the two pedicles. With the bone cage in place, the pedicle screws are compressed along the rods which will shorten the posterior column, and the bone cage will maintain the anterior column height by keeping the disc space distracted, thereby restoring lordosis of the lumbar spine. The rods are then tightened to the screws to hold the spine in its new position until the bone graft fuses. The bone cage and pedicle screws have stability (the implant in front, and the two screws in back) forming a triangle.
Prior to fusion, and prior to inserting the bone cage, pedicle screws absorb the stress of supporting the spine. In older patients, bone is weaker and the pressure of the pedicle screws can cause osteoporotic complications. Thus, it is desirable, when possible, to effectively balance the pressure placed on the pedicle screws and on the bone cages. This requires that the bone cage make effective contact with the endplate of the two vertebrae bodies during the fusion process.
There are many versions of bone cages known in the art, and many attempts have been made to solve the problem of stable placement of bone cages with the optimum and controlled contact with the vertebral endplates. However, devices known in the art are associated with problems due to incomplete or uncontrolled contact between the endplates of the vertebral bodies and the upper and lower surfaces of the cage. Many attempts have been made in the art to create bone cages which can be adjusted or positioned to account for physiological differences in patients and achieve pressure reduction.
Bone cages, such as the Continental™ and Colonial™ by Globus Medical, have a chamfered leading edge to facilitate insertion in multiple footprints, heights, and profiles allowing the surgeon to match various patient anatomies. However, these devices cannot be adjusted for a particular patient nor can they be expanded to provide physiologic lordosis (i.e., normal curvature of the lumbar spine where the disc space is wider anteriorly).
The Sustain® O by Globus Medical has a tapered leading edge for easier insertion and rounded corners, which allow for rotation (positioning) during insertion, and thus also attempt to give the physician greater control. The Continental™, Colonial™ and Sustain® O, however, are not desirable because they are not ideally contoured to the shape of the vertebral endplates resulting in a minimal area of contact between the cage and the vertebral endplates. In addition, each bone cage has a fixed height, which cannot be adjusted after insertion.
Bone cages contoured to mimic the shape of vertebral endplates are also available. Globus Medical also has bone cages with varying shapes designed to mimic the shape of vertebral endplates. The Sustain® Small has a convex sagittal profile to mimic the shape of vertebral endplates, Sustain® Large has a trapezoidal footprint to mimic the shape of vertebral endplates, and Sustain® Medium has a teardrop footprint to mimic the shape of vertebral endplates, The LT-Cage® Lumbar Tapered Fusion Device by Medtronic is tapered to more closely match the shape of the disc space.
The AVS TL PEEK and AVS PL PEEK Spacer Implants by Stryker are also available in a shape that more closely mimics the shape of the vertebral endplates. Each is available in rectangular or parallel (0 degree) and wedge (4 degree) as well as in varying heights and two different lengths/widths. The Ogival Interbody Cage implant by Stryker is a bullet shaped cage to facilitate intracanal navigation and is available in 4 degree and 8 degree lordotic versions to provide better coverage. In addition, the AVS TL PEEK, AVS PL PEEK, and Ogival Interbody Cage implants include serrations on the top and bottom weight bearing surfaces to provide stability and prevent migration. Each of these bone cages is uniquely shaped and designed to mimic the contours of the vertebral endplates, however, the correct bone cage and the correct size/height must be chosen in order to match each patient's particular anatomy. None of these bone cages are capable of being adjusted to conform to a particular patient's vertebral endplates.
Other types of expandable bone cages, such as the VBoss Implant by Stryker are also expandable, The VBoss Implant has an expandable column with modular end caps that are available in 5 diameters with 0, 5, or 10 degree angles to enhance restoration of lordsis. The XPand® by Globus Medical is an expandable cage that comes in a variety of footprints, heights, and lordotic angles. Both of these devices can be expanded vertically; however, these cages are indicated in cases to replace the whole vertebra body after the entire vertebral bone is removed.
U.S. Pat. No. 6,852,129 (Gerbec '129) teaches a wedge-shaped bone fusion implant with expandable sidewalls that allows the height of the implant to be adjusted when a component is inserted for expansion. This design requires that the physician manually control expansion and determine the position of the plates of the device without mechanical guidance or physical precision. In addition, the flattened and rectangular plates of the device do not accomplish effective contact with the endplates, and this leaves a gap between the surfaces of the device and the endplates.
U.S. Pat. No. 6,962,606 (Michelson '606) teaches an adjustable “push in” implant by which the “front, back or both” of the implants are raised by “the same or various amounts.” The implant taught by Michelson '606 is expandable; however, Michelson '606 is not enabling and does not teach one of ordinary skill in the art how to make or use a bone cage.
Several problems are associated with this device. Michelson '606 does not enable a device which effectively makes contact with the endplates of the vertebral bodies because the device is comprised of two flattened (or uniformly curved) panels that do not conform to the contours of the vertebral bodies and thus the objective of the device of reducing pressure on the pedicle screws and equalizing the pressure over the surface of the device is not effectively achieved despite the capability of the device to be expanded in place. More significantly, placement of the Michelson '606 device requires a rectangular “blocker” component to keep the ends apart and the expansion is not effectively controlled. In addition, this device has only two positions: open or closed.
Michelson teaches expanding the implant with an unspecified tool “such as a spreader or distracter [or] scissor type” device. No method for expanding the device is disclosed or claimed, and a “scissor type” device cannot provide adequate control, which is critical during placement of the cage. Michelson '606 does not allow for controlled expansion of the device, which can result in substantial risk to a patient because the bone cage may pack or break through the bone of the vertebral bodies.
The device disclosed in Michelson may potentially be dislodged because it does not have secure points of contact. In addition, the lack of secure points of contact results n an unpredictable reduction of pressure over the pedicle screws.
It is desirable to have a bone cage that allows for maximum control and predictability of expansion, and which protects the endplates from fracture by distributing the forces along a greater surface area.
It is desirable to have a bone cage that is in secure contact with the endplates of the vertebral bodies.
It is desirable to have a modular system that allows the surgeon to size the implant to accommodate patients with varying size disc spaces and/or disc heights.
It is desirable to have a reliable means for expanding a bone cage device.
It is desirable to have a bone cage which reduces pressure on pedicle screws during the fusion process by maximizing contact between the bone cage and the vertebral endplates through the use of anthropometric contours.
It is desirable to have a bone cage which is shaped to conform to the contours of the vertebral endplates and which is capable of expanding at an angle optimum for controlled support and to prevent the device from being displaced while restoring lordosis to the spinal column.
It is further desirable to have a bone cage which is adapted for insertion of bone graft material after the device is stably in place, minimizing the risk that the bone graft material will be displaced or come in contact with the nerves during implantation.